The present invention relates generally to lasers and more particularly to an annular lasing apparatus especially well-suited for the uniform irradiation of flowing fluids.
Laser chemistry and laser processing of materials has received much attention from chemists and laser technologists for several years. Among the difficulties in performing efficient irradiations of fluids in continuous processing situations is the pencil-shaped transmission characteristics of the laser output. That is, mixtures of materials intended for laser-induced reaction are generally transported through cylindrical pipes in which irradiation is also convenient. To maximize the interaction of photons and molecules, irradiation along the direction of flow would provide the longest pathlength. However, this is unusually difficult to arrange in most situations. Irradiation normal to the flow, even with multiple reflection arrangements, cannot result in uniform irradiation of all of the material in the reaction region. Clearly, a radial irradiation geometry would be more efficient since the flowing fluid could be illuminated with substantial uniformity about its entire circumference, and processes can be envisioned where an intense, uniform irradiation region having a short length in the direction of the flow of materials would be advantageous.
Additionally, a radial irradiation geometry would be of value in ionizing streams of molecules or atoms for introduction into ion acceleration systems.
Cylindrical laser resonators have been described by L. W. Casperson in "Cylindrical Laser Resonators," J. Opt. Soc. Am. 63, 25 (1973), and by L. W. Casperson and C. Romero in "Properties of a Radial Mode CO.sub.2 Laser," IEEE J. Quant. Electron. 9, 484 (1973). U.S. Pat. No. 3,940,711, "Cylindrical Laser Resonator," issued to L. W. Casperson on Feb. 24, 1976 combines the teachings of both of the above-referenced publications. The first of these references teaches a disk resonator in which the radiation propagates partially in the radial direction. The author states that the properties of such resonators include high field intensity and uniformity of illumination near the axis of the resonator, and that applications might include the excitation, vaporization, ionization, or fusion of samples. However, the analytical expressions given for the radial dependence of the laser energy demonstrate that this energy is sharply peaked at the axis. The energy is uniform about the axis though. In the second reference, the authors teach the use of a mode suppressing aperture within a disk-shaped amplifying medium to reduce the angular dependence of the mode structure, thereby increasing the number of radially propagating modes. It is stated that the resonator described therein provides an extreme focusing of the laser energy at the axis, albeit symmetrically disposed thereabout. In some processes, however, where the irradiated sample has a finite diameter (not extremely small), it would be advantageous to have an intense, substantially uniform in the radial direction, and short length in the direction of the axis heating zone which cannot be obtained from the lasers taught in the above-mentioned references.
The patent to Casperson discloses both disk and tube configuration lasers, but claims only the tube configuration. The first and third of the above-mentioned references describe the use of a fixed position, continuous, concentric inner partially reflecting mirror to extract the laser radiation in the region of the axis of the disk resonator, while the second and third references teach the use of a small mirror in the shape of a truncated cone that is located at the axis position to skim a part of the laser energy from the resonator and reflect it out through a hole at the top of thereof. There are no teachings of the use of other than a fixed position, continuous cylindrical outer mirror in cooperation with the above-described output coupling mirrors in order to achieve the disk laser properties described.
The conditions for stability for an optical resonator include the radii of curvature of the mirrors, their spacing and the indices of refraction of the laser gain medium and the materials which are to receive the laser radiation, this latter index of refraction becoming important for resonator configurations where the irradiated material is inside of the laser cavity. In An Introduction to Lasers and Their Applications, by D. C. O'Shea, W. R. Callen and W. T. Rhodes, Addison-Wesley Publishing Company, Reading, Mass., 1978, a simplified description of the stability criteria is given in chapter 3. In particular, FIG. 3.10 (b) shows that the confocal resonator design utilized by Casperson in the references described hereinabove is only marginally stable at best. Although such resonators are useful under certain circumstances, they can be used only with an active medium that has a high small-signal gain, for example, carbon dioxide or carbon monoxide, which places severe limits on their useful wavelength range. In order to design a resonator which satisfies the conditions of stability, it is necessary to have some adjustable parameters such as the radii of curvature of the mirrors and their relative positions, and in general, the problem of excessive diffraction losses is overcome by curving the mirrors slightly inward toward the cavity; that is, by making them concave inward. Moreover, micrometer adjustment of the relative positions of laser resonator mirrors, as is commonly practiced in the field, reduces the machining tolerances required for an operable resonator. In the Casperson resonator design, the mirror radii are fixed by the dimensions of the disk, and the inner mirror is inwardly convex. Indeed, the only disk laser described as having been constructed and tested by Casperson is the radial mode TEA laser with the on-axis output coupling mirror to couple a portion of the laser radiation out of the resonator as shown in FIG. 5 of his patent and in FIG. 3 of the second of the journal publications described hereinabove. With the obvious absence of any inner internal reflectors, this embodiment of Casperson's resonator is stable, and thus more readily made operable.